Storage devices typically include a head for reading and/or writing data on to a storage medium. A suspension assembly is required to position and maintain the head at the appropriate position relative to the storage medium for data storage and/or access. Most common today are magnetic heads and magnetic storage media, particularly flexible and rigid disks.
The present invention is directed to a suspension assembly of the type that is typically used for supporting a head relative to a rotating disk wherein the head flies over the disk surface when accessing the disk on an air bearing. Such is the case in most rigid disk drives. Specifically, the magnetic head is usually provided on a slider having an aerodynamic design so that the movement of air caused by a spinning disk will generate a lift force against the slider which prevents contact or crashing of the head with the disk surface. Preferably, the air bearing is minimized while sufficient to accommodate disk fluctuations since storage density is decreased significantly as the height of the air bearing increases.
In order to counteract the lift force acting against the slider, the suspension assembly typically includes a load beam having a resilient section and a rigid section; the resilient section providing a spring force to the rigid section for urging the slider in a direction opposite to the lift force. The "flight" of the slider and head is a balance of the lifting force and the opposing spring force.
A spring or gimballing connection is also typically provided between the slider and the rigid section of the load beam so that the slider can move in the pitch and roll directions of the head for accommodating fluctuations of the disk surface. Such a spring connection can be provided by a gimbal that is made separately and connected with the rigid region of the load beam or is made integrally at the end of the load beam. The spring connection of the gimbal is also preferably designed with high lateral and in-plane stiffnesses. Such gimbals, in general, are well-known.
In addition to providing the aforementioned spring force, the load beam must also provide the rigid link between an actuator of the disk drive and the slider/head assembly for precisely positioning the head relative to data tracks of the storage medium, i.e. the disk surface. Actuator movement is normally either rotary or linear over the disk surface. The ability to provide such a rigid link is adversely affected by the confined geometry constraints of today's disk drives, i.e. disk sizes, disk spacing, data storage densities and access speeds, attachment methods, and low stiffness of the suspension in the Z-height (direction perpendicular to the plane of the disk surface). As the rigid linkability of a load beam is compromised by these other design features, the load beam must otherwise be designed to increase rigidity to prevent off-track errors.
Given these design criteria combined with disk and access speeds, the suspension assembly can be subject to high vibration frequencies which can cause off-track error, particularly at resonance frequencies of the suspension assembly, if not controlled. It is thus an important additional design criteria of suspension assemblies to design the suspension assembly so that its resonance frequencies are higher than the frequencies experienced in the drive environment or to minimize the gain (movement of the suspension assembly at the slider) caused at the resonance frequencies. Of most concern in the design of suspension assemblies are the resonance frequencies of the torsional and lateral bending modes. These torsional and bending modes are beam modes that are dependent on cross-sectional properties along the length of the load beam. These modes also result in lateral movement of the slider at the end of the suspension assembly. Torsional modes sometimes produce a mode shape in which the tip of the resonating suspension assembly moves in a circular fashion. However, since the slider is maintained in a Z-height orientation by the stiffness of the applied spring force acting against the air bearing, only lateral motion of the rotation is seen at the slider. The lateral bending mode is mainly lateral motion.
The lateral bending mode is normally controlled by the design of the cross-section of the load beam, i.e., side rails, channels, and the like. Typically, the resonance frequency of the lateral bending mode is controlled by the load beam design to be higher than the frequencies that are experienced in the disk drives within which they are used. Certain torsional resonance modes, however, occur at lower frequencies, but typically have less of a lateral effect. Torsional modes are further subdivided depending on the number, if any, of nodes present along the length of the suspension assembly between a fixed end thereof and a free end, the slider being supported near the free end. These various torsional mode shapes occur at different resonance frequencies. A single twist of the suspension assembly between the fixed end and the free end, as used throughout this specification, is referred to as first torsion mode, and the off-track motion at the first torsion resonance frequency is referred to as the first torsional gain. Second torsion mode, as used throughout the specification, means a torsional mode shape having a single node along the length of the suspension assembly between its fixed end and its free end. The position of the node divides the suspension assembly into first and second twisting motions on either side of the node point. The second torsional resonance frequencies occur at higher frequencies than the first torsional mode. Second torsional gain refers to the amount of off track motion when the suspension assembly is vibrated at the second torsional resonance frequency. Typically, design consideration must be given to the first and second torsional modes under the vibration frequencies normally experienced within a disk drive environment. Higher order torsional modes, i.e. third torsional mode having two node points, etc., typically occur at frequencies higher than that experienced within the disk drive environment.
Since torsional modes have a twisting movement from the fixed end of a suspension assembly toward its free end, each torsional mode shape (first torsional, second torsional, etc.) defines an axis of rotation along the suspension assembly. The axis of rotation need not, and in fact generally does not, lay within the suspension assembly.
Moreover, it is preferable that the axis of rotation be defined such that it runs through or near a gimballing location of the slider and the load beam of a suspension assembly. If the axis of rotation can be controlled to pass through or near the gimballing point, minimal off-track movement is experienced at the slider. In other words, even if the resonance frequency of a particular torsional mode is experienced in the disk drive environment, the gain associated with that resonance frequency can be controlled to be minimized. It is thus a significant endeavor in the design of suspension assemblies to minimize the gain associated with first and second torsional modes as they typically may be experienced in a disk drive environment.
The mass of the suspension assembly and how that mass is distributed along the suspension assembly has a large impact on the suspension resonance frequency and gain characteristics. For example, the addition of mass at a location of maximum displacement depending on the mode (bending or torsional) can be used to reduce the natural frequency. Although the natural frequency may be lowered, gain may disadvantageously be increased.
In providing the aforementioned spring force to the rigid section of the load beam for counteracting the aerodynamic lift force against the slider, a preformed bend is made in the resilient section of the load beam. This preformed bend is designed with a specific radius to provide the spring force and thus a desired gram loading to the slider when in flight over a disk surface. The term loaded, as used hereinafter, means the suspension assembly in equilibrium under the influence of the aerodynamic lift force and the oppositely acting spring force with the slider at "fly" height.
A manner of optimizing a suspension assembly so that the axis of rotation passes through or near the gimballing point when the suspension assembly is loaded is to modify the suspension assembly to change its loaded profile as represented at its longitudinal centerline, hereinafter "part profile." One way of modifying the part profile is by changing the location of the preformed bend radius area within the spring region of the load beam. By incrementally bending a series of similar suspension assemblies at various locations along the spring area and subjecting each of the suspension assemblies to its torsional resonance frequencies, the gain associated with each bend location can be measured. The measured gain can be plotted against the incremental position of the bend taken from a reference location to determine a curve. At the point where the curve approaches or reaches zero gain, if possible, the axis of rotation is at or nearest to the location of gimballing. In other words, the part profile of the suspension assembly is modified by the position of the radius area of the preformed bend preferably to the point where the axis of rotation passes near or through the gimballing location.
However, on some suspension designs, the minimum point on the curve is not obtainable with the preformed bend because of either process abilities or other design constraints or features (i.e., spring force) which affect the suspension's mass distribution. Additionally, even if the above optimization process can be utilized to minimize first torsional gain, a similar optimization curve representing second torsional gain may not coincide. That is, the location of the radius area of the preformed bend which minimizes first torsional gain may be different from the location which minimizes second torsional gain.
Another manner of controlling the part profile of a suspension assembly is to increase its in-plane stiffness and rigidity by forming its spring region at a precise curvature so that when loaded, the curve becomes shaped to maximize the in-plane stiffness. One prior art method of designing the spring region of a load beam for that purpose is described in U.S. Pat. No. 5,065,268 to Hagen. In this case, two preformed bends are made in the spring region of the load beam in the same bend direction. The preformed bends are precisely located and formed as determined by complex calculations so that the spring region nearly assumes an ideal curve, as defined therein. The object being so that when loaded, the spring region becomes nearly flat, which is utilized to increase the in-plane stiffness.
Another prior art example of designing a specifically curved spring region is described in U.S. Pat. No. 5,471,734 to Hatch et al. According to this method, a single curve side profile is determined theoretically to provide a desired bump and offset characteristics (loaded shape of spring region) and to give the desired load. The spring region must be precisely curved over its length by forming over a forming die having a forming surface calculated to provide the theoretical side curve profile after formation and springback.
In both of the above cases, special calculations and dies and/or forming steps must be determined for each variation of suspension assembly made in order to make a precise curve or approximation thereof for each application.